1. Field of the Invention
The present invention relates to an image forming apparatus such as copiers and printers using an electrophotographic process or electrostatic recording process, and more particularly, to a color image forming apparatus capable of obtaining a full-color image or a multi-color image by transferring a plurality of visible images (also called toner images) of different colors to the same transfer material so as to be superimposed sequentially.
2. Description of the Prior Art
Conventionally, for example, an electrophotographic color image forming apparatus as disclosed in Japanese Unexamined Patent Publication JP-A 5-333708 (1993) has been widely used as a color image forming apparatus. This related art apparatus shown in FIG. 7 will be briefly described.
In the electrophotographic color image forming apparatus of FIG. 7, an electrophotographic photoreceptor drum 74 as an image carrier is uniformly charged by a primary charger 84 realized in the form of a roller or a corona charger. Then, an electrostatic latent image of a first color which is formed on the photoreceptor drum 74 through exposure 86 on the basis of an image signal of the first color from an exposure unit having a light emitting device 83 such as a laser device or a LED (light emitting diode), is developed into a real image by a developer unit 72a containing a developer such as a yellow (Y) developer.
Image formation by a color image forming apparatus will be described with reference to FIG. 7. First, a transfer material 71 supplied from a transfer material conveyance path is sandwiched between a transfer drum 73 and an attraction roller 85 by the attraction roller 85 which is separated from and brought into contact with the transfer drum 73. At the same time, a DC voltage as a bias for attraction and transfer of the first color is applied to a drum cylinder 73a, to electrostatically attract the transfer material 71 to a peripheral surface of the transfer drum 73 and hold thereon, by an electrostatic attraction force of the electric charge from the attraction roller 85 excited by the voltage application.
At this time, the amount of electric charge injected into the transfer material 71 is smaller than the amount of electric charge of the case where the electrostatic capacities of the transfer material 71 and a flexible sheet 73b are sufficiently charged by the same bias. This is because, since the transfer drum 73 carrying the transfer material 71 is rotating when the electric charge is injected into the transfer material 71 by the attraction roller 85, a time during which the attraction roller 85 supplying electric charge is in contact with the transfer material 71 is short, and therefore the supply of electric charge is insufficient, so that in accordance with the incomplete supply of electric charge the surface potential of the transfer material 71 which has passed the attraction roller 85 exhibits the polarity of the bias applied to the transfer drum 73. Therefore, only by setting the polarity of the bias applied to the transfer drum 73 to a polarity that facilitates the transfer of the visible image on the photoreceptor drum 74, transfer can be excellently performed by setting the bias to an appropriate value.
The transfer material 71 held on the transfer drum 73 by the electrostatic attraction force as described above is conveyed to an image transfer section 90 by the rotation of the transfer drum 73 and the visible image of the first color formed on the image carrier 74 is transferred. Then, in the case where a visible image of a second color is transferred, the above-mentioned bias value is changed to correct the surface potential of the transfer material 71 on the transfer drum 73 which surface potential has dropped because of the transfer of the visible image of the first color thereto. When visible images of third and fourth colors are transferred, such correction is also carried out similarly, and the visible images formed on the photoreceptor drum 74 are transferred to the transfer material 71 on the transfer drum 73 so as to be superimposed sequentially on one another.
The transfer material 71 which has undergone the above-described transfer process is diselectrified by a separation charger 79, so that the electrostatic attraction force acting between the transfer material 71 and the transfer drum 73 is removed. The transfer material 71 which is carried under this state is separated from the transfer drum 73 with the separation discharge restrained by a separation diselectrifying charger 80. Then, the visible image formed on the transfer material 71 is fixed by a fixing unit 76 into a permanent image. Reference numeral 82 represents a sheet diselectrifying charger. Reference numeral 88 represents a power source for attraction bias.
In such a related art apparatus, the attraction force of the transfer material 71 is obtained by the Coulomb's force depending on the electric charge on the transfer material 71 and the voltage applied to the drum cylinder 73a of the transfer drum 73. Therefore, when electric charge of the same polarity as that of the voltage applied to the drum cylinder 73a of the transfer drum 73 is supplied to the surface of the flexible sheet 73b, the attraction force of the transfer material 71 decreases even if the value of the voltage applied to the drum cylinder 73a of the transfer drum 73 remains the same. The surface of the flexible sheet 73b is supplied with electric charge by the contact between the surface of the flexible sheet 73b and the photoreceptor drum 74.
Since the electrophotographic photoreceptor drum 74 using a laser exposure system normally employs the reversal development method, the peripheral surface of the photoreceptor drum 74 is charged to the same polarity as that of the toner. Therefore, in order to transfer toner, a voltage of the opposite polarity to that of the toner is applied to the drum cylinder 73a of the transfer drum 73. For example, when the toner is negatively charged, the peripheral surface of the photoreceptor drum 74 is also negatively charged, and a positive voltage is applied to the transfer drum 73.
Consequently, positive electric charge moves from the photoreceptor drum 74 to the flexible sheet 73b. When the voltage applied to the transfer drum 73 is high, the potential difference between the photoreceptor drum 74 and the transfer drum 73 is large, so that the amount of electric charge supplied from the photoreceptor drum 74 to the flexible sheet 73b increases. The surface of the flexible sheet 73b is initialized by the sheet diselectrifying charger 82 immediately before the transfer material 71 is held on the transfer drum 73. Particularly, when a high transfer voltage is used, for example, in the case of transfer on an OHP (overhead projector) sheet, electric charge is supplied from the photoreceptor drum 74 not only to the surface of the flexible sheet 73b but also to the inside of the flexible sheet 73b.
For this reason, the related art image forming apparatus has drawbacks that the initialization of the potential of the transfer drum 73 by the sheet diselectrifying charger 82 is insufficient, and although the surface potential does not largely vary when image formation is performed only on one transfer material, the surface potential of the transfer drum 73 gradually decreases when image formation is continuously performed, so that not only excellent transfer can not be realized, but also the transfer material 71 cannot be held on the transfer drum 73.